The present invention relates to conductive magnetic microparticles, their formation, and their use in a matrix of electrically nonconductive materials to produce an electrically conductive composite.
It is known that the inclusion of conductive metal particles in a matrix of electrically nonconductive material will produce a composite which is electrically conductive. Such composites rely upon particle-to-particle contact to produce a network of particles through which electrical current may flow. Typical composites rely upon particles of metals such as copper, tin, silver, gold, platinum, stainless steel, nickel, and various alloys thereof, or other conductive materials such as carbon or carbon fibers. These conductive particles have been mixed into polymers such as acrylics, phenolics, alkyds, rubbers, silicones, vinyls, urethanes, and other electrically nonconductive materials.
However, these composites suffer from one or more of the following detrimental characteristics: (1) In materials utilizing metals as the conductive component, the metal particles are very dense compared to the polymer matrix and thus tend to settle and separate from one another both before and during usage. (1) In materials utilizing carbon as the conductive component, the amount of carbon required to produce desirable conductivity causes the resulting composite to lose desirable mechanical properties. (3) In compositions which do achieve good conductivity the quantity of particles that must be incorporated make them very heavy.
It has also been proposed to use silver coated particles, such as silver coated glass fibers, glass flakes, solid glass spheres, and hollow glass spheres, in resin composites to render the composites electrically conductive. Heinze and Ritter, "Effects of Particle Type on the Conductivity of Silvered Glass in Reinforced Composites", Annual Technical Conference, 1977 Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc. However, these composites were reported to suffer many of the same problems noted above. The heavy silver coated solid glass particles tend to settle and give inconsistent electrical conductivity properties throughout the composite. The lighter silver coated hollow glass spheres were reported to require a high percentage (25% by weight) silver and to produce significant strength loss in the composite. The hollow glass spheres are quite fragile and are easily broken during processing. Additionally, difficulties have been encountered in obtaining a uniform coating with good adherence. Furthermore, the hollow glass spheres are relatively costly, and this, coupled with the high percentage silver required, limits their usefulness for many applications.